Many states have adopted Renewable Portfolio Standards (RPS) that require electricity supply companies to increase energy production that is attributed to renewable energy sources. The federal government may soon implement a renewable electricity standard (RES) that would be similar to the “renewables obligation” imposed in the United Kingdom. These standards place an obligation on electricity supply companies to produce a specified fraction of their electricity from renewable energy sources, such as wind, solar, hydroelectric, geothermal, biofuels, and biomass.
“Biomass” refers to renewable organic materials such as wood, forestry waste, energy crops, municipal waste, plant materials, or agricultural waste. Biomass often contains about 10 to about 50 weight percent moisture and up to about 50 weight percent oxygen. The moisture and oxygen content in raw biomass decreases its fuel value and increases costs associated with transportation of the biomass. Thus biomass is a low grade, high cost fuel that cannot compete economically with fossil fuels most commonly used to generate electricity. Further, biomass has a low bulk density, is very hydrophilic, can be seasonal, is variable, and has a limited shelf life.
“Torrefaction” refers to the processing of biomass at temperatures between about 200° C. to about 350° C. (400°-660° F.) at atmospheric pressure wherein water and light volatile organic chemicals associated with the raw biomass material (i.e., “feed stock”) are vaporized. In addition, during the torrefaction process, molecules of biopolymers (hemicelluloses and some lignin) contained in the biomass decompose. After torrefaction, the biomass is a solid, dry, blackened material that is often referred to as “torrefied biomass” or “biocoal” that is easier to grind than raw unprocessed biomass and has a heating value comparable to low-rank coals, which allows it to be used in coal burning power plants without any major equipment modifications or plant capacity derate. Further, the torrefied biomass has a significantly, reduced moisture content, and has higher fixed carbon levels.
Fluid bed reactors are commonly used to carry out multiphase reactions. In this type of reactor, gas or liquid is passed through a granular solid material at high enough velocity to suspend the solid and cause it to behave as though it were a fluid. This process, known as “fluidization” imparts many important advantages to the reactor. As a result, the fluidized bed reactor is now used in many industrial applications, such as coal drying. Commonly coal drying is performed in an inert gas, i.e., oxygen-free environment. Drying coal in a non-oxidizing environment requires external heat sources to maintain the temperature of the reactor. However, coal has been dried in an oxidizing environment where the heat used to support the process is at least partially drawn from the burning coal. The temperature of the fluid bed reactor used to dry and otherwise process the coal is controlled by balancing the rate at which the coal is fed into the reactor against the amount of heat generated by the combustion process. Drying of coal increases the heating value of low rank coals, reduces the particle size of the feed stock, and partially decarboxylizes and desulfurizes the coal. After the coal is dried, it must be rehydrated to raise the moisture content up to about 5-9% to reduce its spontaneous combustion characteristics so that it is similar to native coal.
The table provided below illustrates the differences between raw coal and processed coal. One of skill in the art will appreciate that processed coal possesses a higher fixed carbon and heating values correspond to raw coal and the moisture content is drastically reduced.
Raw CoalProduct 1Product 2Product 2Proximate Analysis:Moisture20.16% 8.00% 8.00% 8.00%Ash 8.16% 7.93% 8.69% 8.67%Volatile Matter31.70%35.33%34.90%35.05%Fixed Carbon39.98%48.74%48.42%42.48%Ultimate Analysis:Moisture20.16% 8.00% 8.00% 8.00%Hydrogen 2.87% 3.32% 3.19% 3.14%Carbon55.50%63.15%62.65%62.74%Nitrogen 0.75% 0.99% 1.12% 0.81%Sulfur 0.77% 0.52% 0.54% 0.48%Oxygen11.79%16.09%15.82%16.16%Ash 8.16% 7.93% 8.69% 8.67%Heating Value, Btu/lb9,44410,46010,31510,165
Fluidized bed processing of biomass has similar effect on the biomass product (biocoal). Further, for certain straw-like biomass, and possibly woody biomass residue, it is advantageous to pelletize biomass before torrefaction. Resulting torrefied product has bulk properties similar to that of coal, i.e., grinds easier and is easier to handle. Often, however, a binding agent, such as lignin, which also naturally occurs in the biomass, must be added to pulverized raw biomass so that pellets or briquettes (used in some instances interchangeable herein) can be formed. Often, pellets are formed in extrusion operation whereas briquettes are made in a pressing operation. The following patents and published patent applications describe raw biomass pelletizing processes of the prior art.    1) US 2008/0190015 A1—Describes that pellets can be formed from raw biomass on the basis of pressure with no regard to lignin content of the biomass.    2) U.S. Pat. No. 4,308,033—Describes grinding biomass to an unspecified size, drying the biomass to 13% moisture or less, adding a wax-water emulsion or a wax-lignin mixture to provide supplemental binder, adjusting the binder-biomass mixture moisture to approximately 28% moisture, and pelletizing the material in a conventional pellet press.    3) “Densification characteristics of corn cobs”, N Kaliyan and R Morey—This study found that corn cobs at a moisture content of approximately 10% would form a good quality briquette/pellet at a temperature of approximately 85° C. (185° F.) and a pressure of 150 MPa (21,756 psi).    4) “Natural binders and solid bridge type binding mechanisms in briquettes and pellets made from corn stover and switchgrass”, Bioresource Technology 101 (2010) 1082-1090, N. Kaliyan and R. Morey—This publication studied the binding mechanisms of natural binders found in corn stover and switch grass and found that natural binders softened at a temperature between 50° C. and 113° C. (122° F. and 235° F.). Moisture contents were in the range of 10% to 20%. Material tested was ground to corn stover particle size equal to about 0.34 mm and switch grass particle size equal to about 0.49 mm.    5) US 2009/0205546 A1—Describes that pellets can be made using a mixture of at least 75 weight percent cellulosic materials (biomass) and 5 to 15 weight percent thermoplastic polymeric materials. The temperature of the thermoplastic polymeric material may be raised to between 140° C. and 145° C. (284° F. and 293° F.) before or during mixing. The mixture is then pelletized and cooled. Thermoplastic polymeric materials are described as polyethylene, polypropylene, polyamide, polyimide, or combinations thereof    6) US 2010/0206499 A1—Describes the conversion of a pulp processing plant to a biomass based pellet manufacturing plant. The biomass feed is first processed by a convention pulp digester. This digester liberates the lignins present in the biomass. The pulp from the digester is then recombined with the liberated lignins and then pelletized. No pelletizing conditions are described.    7) “Feedstock and Process Variables Influencing Biomass Densification”, Mark Shaw Master of Science Thesis—This document evaluated the affect of pressure and temperature on biomass pellet quality. Test work was completed using poplar wood and wheat straw and included various pressures between 31.6 and 126.3 MPa (4.6 and 18.3 psi) and temperature between 70 and 100° C. (158 and 212° F.). Moisture contents of the feed were 9 and 15%. This work also made an evaluation of the effect of pre-treating the biomass with steam expansion. A number of trends were indicated by this work but no definitive direction was offered on how to make a quality pellet. The overall conclusions are that 1) chemical composition of the feed stock did not have a large impact on compression/compaction, and 2) particle size and moisture content have more significant impact on compression and compaction than chemical composition.
Note that most these references are concerned with pelletizing raw biomass, and in some instances pulp, not torrefied biomass. More specifically, torrefaction significantly reduces the presence of hemicellulose compounds, to a lesser degree lignins, and thus torrefied biomass would be expected to pelletize differently than raw biomass or pulp and thus would be subjected to different pelletizing processes and requirements.
More specifically, attempts to pelletize torrefied biomass have revealed that a glassy surface forms on the exterior surface of the pellets when pelletization is performed in the range of about 121 to 135° C. (250-275° F.) as measured after the pellet was discharged from a Bliss pellet mill. Because in one test there was a one or two minute delay between pellet production and because the actual temperature conditions of the pellet press is higher than ambient, it is believed that pelletization temperature may have been higher. The glassy surface appears to be due to surface porosity sealing that imparts hydrophobicity to the pellet which enhances resistance to water degradation.
The pellets produced in one test were made from fines collected by a cyclone used in torrefaction process. The fines appear to be more evenly torrefied than the average processed biomass product consisting of larger particles. The fines were heated to approximately 135° C. (275° F.) and pressed into pellets. The resultant pellet appeared to be much denser than those observed in previous attempts to form pellets at lower temperatures. The pressed pellet did not have a glassy surface that was observed when the final torrefied biomass product was pelletized using ring-die pelletizing machine. No attempt was made to evaluate this pellet to determine its hydrophobic nature or resistance to water degradation, however. One of skill in the art will appreciate that pressed pellet does not have the same surface characteristics as the extruded pellet from a ring-die machine.
The work by others in an attempt to pelletize torrefied biomass appears to be under conditions similar to that used by traditional wood pellet processes, which typically restrict it to temperatures of about 65-93° C. (150-200° F.). However, under those conditions, the pellets are not hydrophobic and tend to degrade when exposed to moisture unless additional binders are added. As one of skill in the art will appreciate, adding binders adds additional production steps and costs.